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CALCPARK

Online Stopwatch & Timer

Online stopwatch with lap times and a countdown timer with an alarm. Keyboard shortcuts, a full-screen display and timestamp timing that stays exact in background tabs.

Runs on your device
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How to use it

  1. Choose Stopwatch or Countdown timer in the tabs.
  2. Stopwatch: press Start (or Space). Press Lap (or L) at each split; Stop pauses, Start again resumes, Reset clears everything. Copy laps exports a tab-separated table.
  3. Timer: type hours, minutes and seconds or tap a preset, then Start. Pause and Resume keep the remaining time; Reset restores the full duration.
  4. When the timer reaches zero the display turns amber, the tab title changes and the alarm sounds – press Dismiss (or Space) to stop it.
  5. Press F or the Full screen button for a large display across the whole screen; Esc leaves it.

Example

Timing a 400 m interval session: Start, then Lap at the end of each repeat. Laps of 01:12.40, 01:10.85 and 01:15.02 give a total of 03:38.27; lap 2 is marked fastest and lap 3 slowest, and the average lap is 01:12.76. For the rest period, switch to the timer, tap 3 minand Start – the alarm beeps at 00:00 until you dismiss it.

How the timing works

stopwatch reading = accumulated time + (now − started at)      while running
timer reading     = end time − now                               while running

Browsers slow down timers in background tabs (typically to once per second, or once per minute after a few minutes of inactivity). A stopwatch that adds 10 ms on every tick would therefore fall behind. This one stores instants instead and recomputes the reading on each tick, on every return to the tab and, for the timer, with an extra one-off timeout scheduled for the exact end, so the alarm fires within a fraction of a second of the right moment even after the tab was throttled.

The display updates every 50 ms (stopwatch) and 200 ms (timer); resolution of the stored values is the system clock’s millisecond. Laps are kept in memory for the page view only – copy them if you need to keep them. The countdown rounds seconds up, so “00:01” means up to one second remains and “00:00” is the end.

Sources: Chrome – timer throttling in background tabs, MDN – Web Audio API, MDN – Fullscreen API.

Questions

Does the stopwatch keep running if I switch tabs or lock the screen?
Yes. The elapsed time is the difference between the system clock and the start instant, so when you come back the display shows the correct value. If the browser suspends the page completely (some mobile browsers do after a long time), the reading is still correct on return because nothing was counted tick by tick.
Why does the alarm not sound?
Browsers only allow sound after you have interacted with the page; starting the timer with a click or key press counts. Check the “Play an alarm sound” box is ticked and the tab is not muted. The alarm repeats for 30 seconds or until Dismiss is pressed.
Can I share a timer?
The link carries the mode and the timer length (for example #mode=timer&t=300 for five minutes), so a colleague opens the page with the same duration ready to start. Running state is not shared.
How accurate is it?
To the resolution of the system clock, about a millisecond; the stopwatch shows hundredths. Reaction time when pressing the buttons – typically 150–250 ms – is the larger source of error in hand timing.